Induced radiative effects such as Raman scattering and fluorescence have become extremely valuable tools associated with the non-destructive determination of molecular constituents. To characterize a composition in a remote or hostile environment, optical fibers may advantageously be used to deliver excitation energy to a sample under investigation and to carry scattered radiation back to means for spectral analysis. An excitation source path may take the form of a laser providing a stimulus at an appropriate wavelength coupled to an input fiber, and a collection path may be made up of a second fiber carrying return radiative information to a spectral analysis tool such as a spectrograph.
Such remote spectral analysis presents technical challenges, however, including the strong scattering signature of the material used for the optical fiber, this interference potentially being generated by both the laser excitation in the illumination fiber and any strong Rayleigh (unshifted) Scattering allowed to enter the collection fiber. These spurious fiber signatures can compete with, or even overshadow, the desired signature of the sample under test, particularly when long lengths of fiber are used.
Referring to the block diagram in FIG. 1, the functional elements of a remote Raman spectroscopy system are schematically illustrated along with the spectral content of the optical energy present at various points along the illumination and collection paths. Energy from an excitation laser 102 is coupled into the illumination fiber 104, beginning as a relatively pure, single wavelength of light as illustrated by the graph 106, which plots intensity as a function of wave number. Upon traversing fiber 104, the laser energy induces Raman scattering within the fiber material, typically composed of silica, yielding a spectrum 108 at the output of the illumination fiber which contains spurious Raman lines in addition to the laser wavelength.
Unless these undesired lines are eliminated from the illumination path before reaching the sample, their Rayleigh scatter at the sample may be indistinguishable from the true, shifted Raman scatter due to the laser excitation of the sample. Therefore, a laser band pass device 110 is used to remove these unwanted wavelengths, thereby outputting, ideally, the single laser line shown in graph 112 to the illumination optic 114 and sample under characterization 120 along path 116. It is assumed for the purposes of this discussion that illumination optic 114 contains a sufficiently short optical path that it does not itself generate significant spurious scattering.
The light scattered by sample 120, depicted by line 122, is collected by collection optic 124. At the output of collection optic 124, as depicted in graph 126, the scattered radiation consists of the unshifted Rayleigh scatter at the laser wavelength and the shifted Raman scatter that characterizes the sample 120 under test. Since the Rayleigh scatter is several orders of magnitude stronger than the Raman scatter, if allowed to enter collection fiber 130, this strong Rayleigh scatter can excite spurious Raman scattering within collection fiber 130 similar to this situation within illumination fiber 104.
This Rayleigh scatter must therefore be rejected before being coupled to collection fiber 130. This may be accomplished with a Rayleigh rejection element 132, which generates the spectra depicted in graph 134, now devoid of the strong Rayleigh line. The collection fiber 130 then conducts only the relatively weak Raman scattering lines, as depicted in graph 134, from sample 120 to an analysis instrument such as spectrograph 140 for detection.
The apparatus of Carrabba et al, U.S. Pat. No. 5,112,127 teaches such a fiber-optic probe useful for measuring Raman spectra of a sample disposed remotely from a light source and detector. This probe head contains optical components which selectively remove unwanted scattering arising from the interaction between the excitation source radiation and the input optical fiber, as previously described, while filtering the Raman excitation source into a return optical fiber leading to a spectrometer or detector.
While effective in certain applications, this prior-art probe includes several inefficiencies in design which limit overall sensitivity and resolution. One drawback concerns the fact that the strong illumination path is substantially in line with the sample, with the weaker radiation of interest being "folded" out of this path. Due to this geometry, the Carrabba probe necessarily relies upon a primary filter in the form of a narrowband transmission element to pass the laser excitation light in one direction and to reflect the Raman signal from its backside. Such an arrangement is prone to inaccuracy for several reasons, the foremost being that this primary filter is called upon to reflect a wide range of desired spectra. With practical filters, however, the efficiency of this reflection varies with wave number. As such, some wavelengths will be attenuated more than others, resulting in an overall signature exhibiting lines of unreliable magnitude. Moreover, this geometry precludes the use of more efficient and accurate optical devices associated with beam redirection and filtering.